Conventionally, a vehicle-mounted electronic control unit (ECU) performs self-diagnosis. When detecting a failure (abnormality), the ECU allows a storage unit to store abnormal information equivalent to Diagnostic Trouble Code (DTC) indicating the failure content and the date and time (time information) of the failure occurrence (e.g., see Patent Document 1).
For example, this type of ECU periodically determines whether an abnormality occurs on a diagnostic object. The ECU counts occurrences of the abnormality detected by the determination process. When the count reaches a specified abnormality confirming threshold value, the ECU first determines that the diagnostic object is abnormal. That is, the ECU detects the abnormality at that point and then stores abnormal information. This aims to prevent incorrect detection of an abnormality.
The abnormality of one diagnostic object may cause the abnormality of another diagnostic object. For example, an oxygen sensor detects the oxygen concentration (or an air-fuel ratio) in the exhaust gas from an engine. While a heater heats the oxygen sensor at an appropriate temperature, the ECU controls the injection quantity supplied to the engine based on an output signal from the oxygen sensor. When the heater causes an abnormality, the oxygen sensor outputs a signal indicating an abnormal voltage value. An abnormality detection process for the heater as a diagnostic object detects the abnormality of the heater. Further, an abnormality detection process for the oxygen sensor as a diagnostic object detects the abnormality of the oxygen sensor.
The technology according to Patent Document 1 stores the abnormal information and the time information about the time to determine the abnormality when a given diagnostic object is determined to be abnormal. When multiple diagnostic objects are detected to be abnormal, the time information stored with the abnormal information can notify the history of the diagnostic objects that are determined to be abnormal.
Generally, however, a time period between the abnormality occurrence and the determination of the abnormality depends on diagnostic objects. When an abnormality of diagnostic object A causes an abnormality of diagnostic object B, diagnostic object B is determined to be abnormal prior to diagnostic object A whose abnormality occurred earlier than diagnostic object B. That is, abnormalities may be detected in reverse order.
The technology according to Patent Document 1 just stores the time information about the abnormality determination and cannot notify such a reversal. When multiple abnormalities are detected, that technology cannot easily find the cause or the first abnormality.
To solve this problem, the technology described in Patent Document 2 stores not only the time information about the abnormality determination but also time information (about the time to start abnormality determination) indicating the time when a state determined to be normal changes to a state determined to be abnormal. When multiple abnormalities are detected, the technology compares the time information about the time to start abnormality determination with the other time information. This makes clear the order of abnormality occurrences and facilitates the estimation of an abnormality cause.
Patent Document 1: JP-H7-181112 A corresponding to U.S. Pat. No. 5,594,646
Patent Document 2: JP-2008-304367 A corresponding to US 2008/0306650
However, the technology according to Patent Document 2 still leaves the following problem unsolved.
The following shows an example where abnormality a of diagnostic object A causes abnormality b of diagnostic object B.
To analyze the relation between two abnormalities a and b, it is necessary to compare pieces of the time information about the time to start determining the two abnormalities a and b with each other. However, the abnormal information or the time information about abnormalities a and b contains no information about the relation between the two. Even though the time information about one of abnormalities a and b is read, the relation about the other abnormality is unknown. The cause of the abnormality cannot be estimated. There may be some reasons why only the time information about one of abnormalities a and b is read. For example, an external tool clears one of the pieces of information. One of the pieces of information is overwritten by information about the other abnormality. One of the pieces of information exists in the other ECU and this ECU is replaced.
According to Patent Document 2, the time information about the most recent time to start the abnormality determination overwrites the time information about an earlier time to start the abnormality determination. It takes time while diagnostic object A is assumed to be abnormal and is finally determined to be abnormal. During this period, the state of diagnostic object A varies. Diagnostic object A is first assumed to be abnormal, then returns to a normal state, and is again assumed to be abnormal. Such state variation is repeated once or more and denotes that the abnormality is indefinite. This method repeatedly updates the time information about the time to start the abnormality determination on diagnostic object A. Let us suppose that abnormality a of diagnostic object A causes abnormality b of diagnostic object B. Despite this, the time information about the time to start the abnormality determination on diagnostic object A is finally stored and may be recorded later than the time information about the time to start the abnormality determination on diagnostic object B. The time information about the time to start abnormality determination does not correctly indicate the occurrence order of abnormalities a and b.